Energy saving, safety and environmental protection are the theme of current vehicle development worldwide, and lightweighting of vehicles plays a very important role. The use of high-strength steels becomes an inevitable trend for the sake of weight reduction and safety. However, an increase in the strength of steel materials generally may lead to a decrease of forming properties, which renders it hard to form a component of complex shape required for vehicle design; meanwhile, springback is a severe problem when cold forming high-strength steels so that it is difficult to precisely control the size and shape of stamped components; and dies are seriously worn during the cold stamping process of high-strength steel materials, which increases stamping costs.
To solve the problem of cold stamping the high-strength steels, a forming method for manufacturing a vehicle component with 1000 MPa or higher strength, referred to as hot stamping or hot forming, is successfully developed and commercially applied on a large scale. The steps of the method comprise: heating a steel sheet into the austenite region of 850 to 950° C.; and putting the steel sheet into a die with a cooling system so as to be formed by stamping at high temperature. At that temperature, the material has a strength of only ˜200 MPa and an elongation of more than 40%, as well as good forming properties, and can be formed into a complex component required for vehicle design, and also has a small amount of springback and high forming precision. The steel sheet is subjected to press hardening at the time of stamping so as to obtain a high-strength formed component of a full-martensite structure.
Bare steel may be oxidized in the course of hot forming, which will affect the surface quality of steel, as well as the die. A conventional steel sheet galvanizing technology, however, cannot meet the conditions for hot stamping process. The U.S. Pat. No. 6,296,805B1 provides a steel sheet coated with aluminium or aluminium-silicon alloy used for hot stamping. Iron in the matrix material may be diffused to the aluminium coating to form an iron-aluminium alloy coating during the hot stamping and heating process. At an austenitizing heating temperature, the iron-aluminium coating will not be oxidized and can effectively protect a steel sheet from oxidization during the entire hot stamping process, and the coating can make a certain improvement in the corrosion resistance of the formed component in service. Therefore it is widely used for commercial purposes. However, in comparison with the conventional galvanized steel sheet, the aluminium-silicon coating cannot provide protection against electrochemical corrosion. The Patent No. EP1143029 provides a method for manufacturing a hot stamped component with a galvanized steel sheet that is formed by coating a hot-rolled steel sheet with zinc or zinc alloy. The galvanized zinc coating, however, has a relatively low melting point of about 780° C., and zinc may evaporate and the zinc-iron coating may melt during the hot forming process, which may result in liquid induced embrittlement and reduce the strength of hot formed steel.
The Patent No. CN103392022 provides a hot stamping technology provided on the basis of a quenching-and-partitioning process, which can realize higher strength and elongation; however, it usually requires that the cooling temperature should be controlled within a range of 100° C. to 300° C., which brings difficulties in controlling temperature uniformity on parts and complication to the production process, and is thus disadvantageous to the actual production of hot stamped components; and the temperature for the austenitizing heat treatment is quite high, which is not good for hot stamping of galvanized sheets and consumes lots of energy.
The Patent No. CN101545071 provides a novel hot stamped steel sheet, wherein the austenitizing heating temperature can be reduced by ˜50° C., which could lead to the reduction of production cost to certain extent. However, the strength-toughness of the hot stamped steel is not significantly improved as compared with the conventional 22MnB5 hot stamped material.
The Patent No. CN102127675B provides an alloy design and stamping method capable of reducing the hot stamping temperature. The method comprises, under the condition of decreased hot stamping temperature, heating a material to a temperature ranging from 730° C. to 780° C., and stamping and cooling the material to a temperature that is 30° C. to 150° C. below Ms point (namely, normally cooled to 150° C. to 280° C.), then further heating the material to a temperature ranging from 150° C. to 450° C. and maintaining the temperature for 1 to 5 minutes to stabilize it to a final state by partitioning carbon from martensite to retained austenite. By applying this method, the ductility of the hot stamped material could be increased on the basis of the Transformation Induced Plasticity (TRIP) effect of retained austenite, but the yield strength of the material is limited below 1150 MPa when the elongation exceeds 10%. In this method, the component must be cooled to a particular temperature ranging from 150° C. to 280° C. before being heated to a temperature ranging from 150° C. to 450° C. and maintained at the temperature, in such a way that the temperature accuracy and uniformity of the component can be hardly controlled, or a complicated production process is required to control the quenching temperature thereof, which is disadvantageous to the actual production of the hot stamped component.